How to control the mix of emission and transparency materials?
As Artturi said, a mix shader can help transition between different shaders.  In the example below, I've used vertex paint to make the black and white image that is used as a factor between a glowing/opaque pink material (mostly an emission shader) and a more transparent purple material (I've used the fresnel node to make the outer edges more opaque, as in the picture you uploaded).

The node setup is more complicated than it needs to be to illustrate the principle, but it does show how you might use it to get something like your picture.